RFR: 8365911: AArch64: Fix encoding error in sve_cpy for negative floats [v2]

[erifan]

On Wed, 3 Sep 2025 08:11:27 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> erifan has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
>> 
>>  - Don't rename sve_cpy as sve_fcpy
>>  - Merge branch 'master' into JDK-8365911
>>  - 8365911: AArch64: Fix encoding error in sve_cpy for negative floats
>>    
>>    The sve_cpy instruction is not correctly implemented for negative
>>    floating-point values. The issues include:
>>    
>>    1. When a negative floating-point number (e.g. `-1.0`) is passed, the
>>    `checked_cast<int8_t>(pack(d))` check fails. For example, assume `d = -1.0`:
>>    - `pack(-1.0)` returns an unsigned int with the 7th bit set, i.e., `0xf0`.
>>    - `checked_cast<int8_t>(0xf0)` casts `0xf0` to an int8_t value, which is `-16`.
>>    - Casting this int8_t `-16` back to unsigned int results in `0xfffffff0`.
>>    - The check compares `0xf0` to `0xfffffff0`, which obviously fails.
>>    
>>    2. Additionally, the encoding of the negative floating-point number is incorrect:
>>    - The imm8 field can fall outside the valid range of **[-128, 127]**.
>>    - Bit **13** should be encoded as **0** for floating-point numbers.
>>    
>>    This PR fixes these issues and renames floating-point `sve_cpy` as `sve_fcpy`.
>>    
>>    Some test cases are added to aarch64-asmtest.py, and all tests passed.
>
> src/hotspot/cpu/aarch64/assembler_aarch64.hpp line 3819:
> 
>> 3817:     if (isFloat) {
>> 3818:       assert(T != B, "invalid size");
>> 3819:       assert((imm8 >> 8) == 0, "invalid immediate");
> 
> Suggestion:
> 
>       assert((imm8 & 0xff) == 0, "invalid immediate");
> 
> To match line 3819.

This may not be the case, `imm8 >> 8` doesn't equal to `imm8 & 0xff`

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PR Review Comment: https://git.openjdk.org/jdk/pull/26951#discussion_r2318181658